Avis Lovelock is a professional sceptic. Using her sharp insights and powers of observation, she sets out to debunk every nefarious spirit medium and psychic she can find. When American businessman Carlton Brooksbank invites her aboard the ocean liner Excambion to take her skills to America, Avis discovers she has been hired to prove to potential buyers that the vessel is not haunted. As she begins her investigations, she runs afoul of the famous twin spiritualists onboard, Gideon and Rachel Duquesne. They are her polar opposites, and Rachel especially has a knack for getting under her skin. The more Avis works to discredit her adversaries and absolve the ship of its haunted reputation, the more she starts to have visions of water filling her cabin, drowned bodies on decks, and flickering lights on the sea. Then someone is found dead. To discover the truth, Avis must join forces with her opponents, and confront some haunting secrets of her own.